Perpendicular magnetic recording medium and process of production thereof

ABSTRACT

A perpendicular magnetic recording medium, which has a low level of recording noise and sufficiently large perpendicular magnetic anisotropy energy relative to demagnetizing field energy, includes a substrate and a multi-layered magnetic film. The multi-layered magnetic film is composed of ferromagnetic metal layers of Co alloy containing at least Cr and non-magnetic metal layers of Pd alloy, each one layer of which are laminated alternately on top of one layer of the other. The ferromagnetic metal layers and the non-magnetic metal layers have a thickness of d 1  and d 2 , respectively, with the ratio of d 1 /d 2  being in the range of 1.5 to 4.0. This specific layer structure reduces the magnetic exchange interaction between magnetic particles in the multi-layered magnetic film. Therefore, the perpendicular magnetic recording medium is stable against thermal disturbance and has a low level of recording noise.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a perpendicular magnetic recordingmedium which comprises a substrate and a multi-layered magnetic filmcomposed of ferromagnetic metal layers and non-magnetic metal layerswhich are laminated alternately on top of the other.

2. Description of Related Arts

The recent advance in information society demands data recordingapparatus, such as hard disk drive (“HDD” hereinafter) , with higherperformance. Improving the performance of HDD is synonymous withincreasing the recording density. One promising technology to meet thisdemand is the perpendicular magnetic recording system which magnetizesthe medium perpendicularly to the film plane, rather than in the plane.There are several candidates as the magnetic material for the recordinglayer of the perpendicular magnetic recording medium. Among them areCoCr-based alloys incorporated with Pt, Ta, B, etc. as additives, whichhave been investigated most closely.

A recording medium of CoCr-based alloy is superior in recordingcharacteristics, but CoCr-based alloy itself has insufficientperpendicular magnetic anisotropy energy (K_(u)) which holds themagnetization in its orientation. For this reason, CoCr-based alloygives an M-H curve with a squareness ratio much smaller than 1 andpermits small reversed magnetic domains to appear in magnetic recordingdomains, thereby reducing the strength of reproduced signals. Eventhough its M-H curve has a squareness ratio close to 1, CoCr-based alloyis still susceptible to “thermal signal loss” which is a phenomenon thatthe state of magnetization changes after recording due to thermaldisturbance.

To cope with this situation, there has been proposed a recording mediumin which the recording layer is a multi-layered film composed of thinfilms (with a thickness of an atomic order) laminated alternately on topof the other. This recording layer can be designed such that themagnetic structure formed by the magnetic head remains unchanged withtime owing to its sufficiently large value of K_(u).

The structure of the multi-layered film mentioned above is generallyreferred to as superlattice structure. It takes on a variety of physicalproperties owing to the state peculiar to the interface between atomiclayers. It is known that the magnetic thin film of multi-layeredstructure has a large perpendicular magnetic anisotropy energy if it isformed from ferromagnetic metal (such as Co and Fe) and noble metal(such as Pd, Pt, and Au) laminated alternately. This multi-layeredstructure of ferromagnetic metal and noble metal is used in theperpendicular magnetic recording medium disclosed in U.S. Pat. Nos.4,587,176, 4,678,721, and 5,106,703.

Unfortunately, the magnetic recording medium with the magnetic thin filmmentioned above has a larger recording noise than the perpendicularrecording medium of CoCr-based alloy. In other words, it is notnecessarily superior in recording properties. The cause of recordingnoise arises from the reversal mechanism of the multi-layered magneticfilm as has been pointed out in many studies.

The fact that CoCr-based alloy thin film is capable of low-noiserecording is because there exist ferromagnetic fine particles in thefilm, with each particle being magnetically isolated by a phase withrich Cr which has spread out on its periphery. By contrast, themulti-layered magnetic film, which takes no special effort to form thefine structure in the magnetic film, permits magnetization reversal totake place in a large area as a unit. Therefore, the magnetic domain forrecording has a zigzag contour regardless of the distribution of themagnetic field applied by the recording head. This zigzag contourreflects the random distribution of the magnetic properties of themagnetic film, and hence it brings about recording noise. To address theproblem involved with the multi-layered magnetic film,. comprehensivestudies have been made on a variety of additives and underlyingmaterials.

Attempts have been made to use a multi-layered magnetic film of Co andPd or Pt as a magnetic recording medium. Its composition, structure, andmanufacturing method intended for better recording characteristics aredisclosed in Toku-Hyo-Hei 11-501755 (Japanese translation of PCTinternational publication WO96/24927). According to this disclosure,each magnetic metal layer is made of Co or Co alloy with a thickness of0.15-1.0 nm and a noble metal layer with a thickness of 0.5-1.5 nm. Thenumber of the magnetic metal layers, each including one noble metallayers, is 10-30. The magnetic recording medium of this structure has acoercive force larger than 2.5 kOe. In addition, according to thedisclosure, the multi-layered magnetic film and its nuclei-forming layer(such as Pd) should have a total thickness smaller than 150 nm so as toavoid an unnecessarily large space between the recording head and thebacking soft magnetic layer (NiFe).

Moreover, an effective way of reducing the noise of recording medium isalso disclosed in the patent publication just mentioned above. Thedisclosure mentions that the recording film should be made by sputteringwith an oxygen-containing sputtering gas at a reduced degree of vacuumand at a high sputtering gas pressure, and annealing should be performedbefore and after the film forming step. The disclosure further mentionsthat it is possible to reduce the recording noise if the Co alloy layeris formed from CoCr or CoCrTa.

Another perpendicular magnetic recording medium with a multi-layeredmagnetic film of Pd/CoCr is disclosed in Appl. Phys. Lett. 64(21) pp.2891-2893 by B. M Lairson et. al. This thesis deals with how therecording characteristics vary depending on the thickness of the CoCrlayer and the amount of Cr added. With the thickness of the Pd layerfixed at 0.4 nm, it concludes that the recording characteristics aresatisfactory if the content of Cr is about 15 at %.

The present inventors repeated the procedure mentioned in the foregoingpatent publication (11-501755) to reproduce the sample No. 12 given inpage 22. This sample is a multi-layered magnetic recording film composedof ferromagnetic layers of CoCr-based alloy and noble metal layers whichare laminated alternately on top of the other. The recording film is ofsuperlattice structure consisting of CoCr layers (0.35 nm) and Pd layers(1.0 nm), with a Pd underlying layer (20 nm). The CoCr-based alloycontains 12 at % Cr. The reproduced sample was actually tested forrecording and reproducing performance. The test results indicate thatthe sample is stable to thermal disturbance but is not particularlysuperior to the conventional CoCrPt recording medium (proposed by Takanoet al., Digest of Intermag 2000, AD-06).

The recording medium reproduced by the present inventors has a higherlevel of recording noise than the CoCrPt medium (just mentioned above)despite incorporation with Cr. A probable reason for this is thatmagnetic exchange interaction between particles is not sufficientlyreduced. On the other hand, the magnetic film has a greatly decreasedlevel of saturation magnetization (˜100 kA/m) because the CoCr-basedalloy contains 12 at % Cr. This results in a significant decrease inreproduced signal intensity. It was found that the S/N ratio of thereproduced recording medium is lower by 10 dB or less than that of theconventional CoCrPt recording medium, when measured with the samerecording/reproducing head due to the increased noise (N) and thedecreased signal (S).

The reason why the reproduced recording signal shows the sufficientlylow recording noise is that Cr in the ferromagnetic layer of CoCr-basedalloy does not fully block magnetic exchange interaction betweenmagnetic fine particles. The laminate structure disclosed in theabove-mentioned patent publication (11-501755) is characterized in thatthe ferromagnetic layer of CoCr-based alloy (of 0.35 nm thick) isrelatively thinner than the noble metal (Pd) layer (of 1.0 nm thick).This thickness ratio is inadequate to reduce the magnetic exchangeinteraction between magnetic particles to such an extent as to suppressrecording noise because there exists the magnetic exchange interactionbetween particles also in the Pd alloy layer due to magnetizationinduced in the Pd alloy layer by the ferromagnetic layer of CoCr-basedalloy. In addition, the small saturation magnetization is apparently dueto the excessively thin ferromagnetic layer of CoCr-based alloy.

However, the above-mentioned report by Lairson et al. shows (in FIG. 5)how the perpendicular magnetic anisotropy energy (K_(u)) depends on thethickness of the CoCr-based alloy layer. It is shown that the K_(u)value decreases with the increasing thickness of CoCr layer to such anextent that it is not useful for the magnetic recording medium.Moreover, in the above-mentioned report, the K_(u) value is merely alittle over 1×10⁵ J/m³ (FIG. 4) in the case of a multi-layered magneticfilm composed of Pd layers (0.4 nm thick) and CoCr layers (0.2 nm thick)containing 15 at % Cr, which gave comparatively good results inrecording and reproducing experiments. The reason for this small K_(u)is the insufficient surface magnetic anisotropy energy (K_(s)).

The foregoing suggests that the magnetic layer for recording cannot beapplied in the practical perpendicular magnetic recording system simplybecause its axis of magnetization is perpendicular to the magneticrecording medium. Tackling this problem motivated the present invention.Accordingly, it is an object of the present invention to provide aperpendicular magnetic recording medium characterized by reducedrecording noise such that the magnetic particles become less susceptiblethermal disturbance.

SUMMARY OF THE INVENTION

In order to achieve the above-mentioned object, the present inventorsstudied the magnetic recording medium having a multi-layered magneticfilm composed of ferromagnetic metal layers containing Co andnon-magnetic metal layers containing Pd which are laminated alternatelyon top of the other. The result suggests that it is possible to obtain aperpendicular magnetic recording medium exhibiting sufficientperpendicular magnetic anisotropy energy with the recording noisereduced below a certain level, if the multi-layered magnetic film has aspecific composition and layer structure and is produced under specificconditions.

The present invention is based on this finding. Thus, the gist of thepresent invention resides in a perpendicular magnetic recording mediumhaving a substrate and a multi-layered magnetic film formed thereon withor without an underlying layer interposed between them. Themulti-layered magnetic film is composed of ferromagnetic metal layerscontaining Co and non-magnetic metal layers containing Pd which arelaminated alternately on top of the other, characterized in that theratio of film thickness defined by d1/d2 ranges from 1.5 to 4.0, whered1 denotes the thickness of each of said ferromagnetic metal layers andd2 denotes the thickness of each of said non-magnetic metal layers whichis between 0.6 nm and 2.0 nm (0.6nm≦d2≦2.0 nm).

The perpendicular magnetic recording medium constructed as mentionedabove may be modified such that the ferromagnetic metal layers areformed from CoCr-based alloy. The resulting multi-layered magnetic filmhas a low level of recording noise due to the reduction in magneticexchange interaction between magnetic particles and also has goodresistance to thermal disturbance due to the sufficient perpendicularmagnetic anisotropy energy to resist the demagnetizing field energyinduced by magnetization of the magnetic film.

The gist of the present invention resides also in a process forproducing the perpendicular magnetic recording medium, which comprises astep of forming a first underlying layer on a substrate which maycontains, ex. Pd, a step of forming a second underlying layer which maycontains, ex. CoCr₄₀, then a multi-layered magnetic film composed ofalternately laminated ferromagnetic metal layers containing Co andnon-magnetic metal layers containing Pd.

The above-mentioned process is preferably modified such that an initiallayer containing a paramagnetic Co alloy interposed between theunderlying layer and the multi-layered magnetic film. In addition, thestep of forming the multi-layered magnetic film is preferably followedby heat treatment in a vacuum at a temperature higher than 350° C.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is illustrated in theaccompanying drawings in which:

FIG. 1 is a cross-sectional view of the magnetic recording medium towhich the present invention is applied;

FIG. 2 is a diagram of a ternary rotary cathode device;

FIG. 3 is a diagram of the multi-layered film formed by a simultaneousdischarging or an alternate discharging with the rotary cathode;

FIG. 4 shows the relation between the Ar gas pressure and theperpendicular magnetic anisotropy energy which is observed during theproduction of the multi-layered film;

FIG. 5 shows how the surface magnetic anisotropy depending on thethickness of Pd film in the magnetic recording medium of the Example 1;

FIG. 6 shows how recording noise depending on the ratio of d1/d2 (whered1 denotes the thickness of Pd layer and d2 denotes the thickness of Coalloy layer) in the magnetic recording medium of the Example 1;

FIG. 7 shows how the thermal stability parameter K_(u)/2πM_(s) ²depending on d1 and d2 in the magnetic recording medium of the Example1;

FIG. 8 is a diagram in which the reduction of reproducing signal isplotted against the elapsed time;

FIG. 9 shows the relation between the thermal stability parameterK_(u)/2πM_(s) ² and the thermal demagnetization factor S in the magneticrecording medium of the Example 1;

FIG. 10 shows the relation between the thermal stability parameterK_(u)/2πM_(s) ² and the ratio of d1/d2 in the magnetic recording mediumof the Example 1; and

FIG. 11 shows how the thermal stability parameter K_(u)/2πM_(s) ²depending on d1 and d2 in the magnetic recording medium of the Example2.

FIG. 12 shows how the thermal stability parameter K_(u)/2πM_(s) ²depending on d1 and d2 in the magnetic recording medium of the Example3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1(a), the perpendicular magnetic recording medium 10according to the present invention consists of a glass substrate 11, anunderlying layer 12 of NiTa₃₇Zr₁₀ alloy, an underlying layer 13 of Pd, amulti-layered magnetic film 14, a Pd cap layer 15, and a carbonprotective layer 16. (Subscripts indicate at %.) The multi-layeredmagnetic film 14 is composed of ferromagnetic metal layers 14-1 andnon-magnetic metal layers 14-2 each one layer of which is alternatelylaminated on top of one layer of the other kind. This perpendicularmagnetic recording medium differs from the sample used for evaluation ofrecording and reproducing which has a soft magnetic backing layer (400nm thick) between the glass substrate 11 and the underlying layer 12.However, there is no difference between them in the magneticcharacteristics of the multi-layered magnetic film.

The multi-layered magnetic film 14 shown in FIG. 1(b) is composed offerromagnetic metal layers 14-1 and non-magnetic metal layers 14-2. Theformer are formed from CoCr-based alloy, and the latter are formed fromPd alloy. The multi-layered magnetic film 14 is characterized in thatthe ratio of d1/d2 ranges from 1.5 to 4.0, where d1 denotes thethickness of each of the ferromagnetic metal layers and d2 denotes thethickness of each of the non-magnetic metal layers.

To incorporate with Cr to reduce magnetic exchange interaction betweenparticles, the ferromagnetic metal layers of CoCr-based alloy(“CoCr-based alloy layers” hereinafter) should be thicker than thenon-magnetic metal layers of Pd alloy (“Pd alloy layers” hereinafter),so that the grain boundary with a high Cr concentration produces astrong effect. Specifically, the ratio of d1/d2 should range from 1.5 to4.0, where d1 denotes the thickness of each of the ferromagnetic metallayers and d2 denotes the thickness of each of the non-magnetic metallayers.

The multi-layered film structure in the perpendicular magnetic recordingmedium 10 exhibits the interface magnetic anisotropy K_(s) and increasesthe perpendicular magnetic anisotropy energy K_(u) only when thethickness d2 of the Pd alloy layer is 0.6 nm or larger. If the Pd alloylayer is excessively thin, the surface magnetic anisotropy becomes smalldespite the multi-layered film structure. If the Pd alloy layer isexcessively thick, magnetic exchange interaction between CoCr-basedalloy layers may be insufficient and individual layers may havedifferent coercive force. Therefore, the thickness of the Pd alloy layershould be 2.0 nm or less. Thus the thickness of the Pd alloy layershould be in the range of 0.6 nm to 2.0 nm, preferably 0.8 nm to 1.2 nm.

The ratio of d1/d2 should be in the range of 1.5 to 4.0. If the ratio ofd1/d2 is smaller than 1.5, there is large magnetic exchange interactionbetween particles and large domain wall displacement at the time ofrecording, which lead to large recording noise. If the ratio of d1/d2 islarger than 4.0, the thickness of each of the ferromagnetic metal layersis large and the effect of increasing perpendicular magnetic anisotropyenergy by K_(s) become negligible. Thus, the multi-layered filmstructure will not improve thermal stability.

In the case of magnetic recording medium with a multi-layered filmcomposed of CoCr-based alloy layers and noble metal layers which arelaminated alternately, it has been common practice to reduce thelaminating period (λ) so as to increase the surface magnetic anisotropyenergy K_(s) induced by the layer interfaces.

Under the condition specified by the thickness ratio of d1/d2, the valueof λ is inevitably large and hence the crystal magnetic anisotropyenergy K_(v) of each CoCr-based alloy layer accounts for a largerportion in the total perpendicular magnetic anisotropy energy K_(u) ofthe multi-layered magnetic films. Therefore, K_(s) merely enhancesK_(u), and it is essential that each CoCr-based alloy layer has a largevalue of K_(v). Therefore, it is necessary to produce the multi-layeredstructure to obtain a high value of K_(v).

One way to increase the value of K_(v) is to keep as low as possible theCr concentration in the CoCr-based alloy layer. Unfortunately, reducingthe Cr concentration in CoCr-based alloy generally tends to weaken theeffect of reducing the magnetic exchange interaction between particlesat the Cr grain boundary. (This is also applicable to monolayerCoCr-based alloys.) Up to now, a Cr concentration of 20% is the minimumamount to reduce the magnetic exchange interaction between particles andto suppress the recording noise.

However, the present inventors found that it is possible to sufficientlyreduce the magnetic exchange interaction between particles in themulti-layered magnetic film 14 even though the Cr concentration isreduced below 20% in the CoCr-based alloy layer, if the followingproduction process is adopted.

-   -   (1) On the underlying layer formed an initial layer (about 3 nm        thick) of paramagnetic CoCr-based or CoCu.    -   (2) On the initial layer is formed the multi-layered magnetic        film 14.    -   (3) After the multi-layered magnetic film 14 has been formed,        heat treatment is performed at 350° C. or above for about 10        seconds.

It was found that if the initial layer contains more than 30 at % of Cror Cu, the magnetic exchange interaction between particles in themulti-layered magnetic film 14 greatly decreases, and consequentlyrecording noise decreases. A probable reason for this phenomenon is thatthe reheating process causes Cr or Cu atoms in the initial layer todiffuse through grain boundary (with high Cr concentration) in thedirection perpendicular to the multi-layered magnetic film 14, and thisdiffusion enhances the effect of decreasing the magnetic exchangeinteraction between particles at clear grain boundaries.

A single-layered recording film of CoCr-based alloy has a large value ofK_(v), but because of low Cr content, it also-has a high value of Ms(saturation magnetization) and hence a high value of demagnetizingenergy. This large demagnetic-energy makes saturation recordingdifficult, and hence it is not suitable for use as the perpendicularmagnetic recording medium.

On the other hand, the multi-layered magnetic film 14 which applies theabove high-Kr CoCr (-based) alloy decreases in M_(s) due to thelamination with the Pd alloy layer and hence the demagnetizing fieldenergy decreases to a proper value. Therefore, the CoCr-based alloy thinfilm, which has a high value of K_(v) (crystal magnetic anisotropyenergy) and a high value of M_(s) (saturation magnetization), isprovided for use as the CoCr-based alloy layer in the multi-layeredmagnetic film 14 according to the present invention.

Moreover, the magnetic recording medium 10 of the present invention ispreferably modified such that the ferromagnetic metal layer in themulti-layered magnetic film 14 contains Pt in addition to Co and Cr.(The Pt-containing metal layer will be simply referred to as a “CoCrPtalloy layer”.)

The incorporation of Pt causes the conventional CoCr-based alloy mediumto increase in perpendicular magnetic anisotropy energy has beenreported many times. The present inventors found that the samephenomenon is also applicable to the multi-layered magnetic filmaccording to the present invention.

In other words, the CoCrPt alloy layer used as the ferromagnetic metallayer in the multi-layered magnetic film 14 has a larger value of K_(u)than the CoCr-based alloy layer. This suggests that Pt added to theCoCr-based alloy layer increases the value of K_(v) (crystal magneticanisotropy energy). Thus, the incorporation of Pt into the CoCr-basedalloy layer is preferred.

It appears that the perpendicular magnetic recording medium exhibitsbetter magnetic characteristics if it has the multi-layered magneticfilm 14 composed of CoCrPt alloy layers (with a low Cr concentration)and Pd alloy layers (non-magnetic metal layers) which are laminatedalternately, and if heat treatment is performed after the multi-layeredmagnetic film 14 has been formed.

A detailed description is given below of the kind and amount of elementsto be added to the multi-layered magnetic film 14. The Cr concentrationin the ferromagnetic metal layer should be in the range of 12 at % to 21at %. As mentioned above, the lower the Cr concentration in theferromagnetic metal layer, the greater the value of K_(v). However, ifthe Cr concentration is lower than 12 at %, the resulting magneticrecording medium has a high level of recording noise due to magneticexchange interaction between particles which cannot be reduced even byheat treatment. By contrast, if the Cr concentration is 21 at %, theresulting magnetic recording medium has an adequate level of magneticexchange interaction between particles even though no heat treatment isperformed.

Also, the Pt concentration in the ferromagnetic metal layer should be inthe range of 10 at % to 16 at %. The Pt added to the ferromagnetic metallayer increases the value of K_(v). The effect of Pt becomes maximumwhen the Pt concentration is higher than 10 at %. However, if the Ptconcentration exceeds 16 at %, the magnetic exchange interaction betweenparticles increases, and the recording noise increases accordingly. Aprobable reason for this is that Pt atoms prevent Cr atoms fromdisplacement toward the grain boundaries.

In addition, it is desirable to add a small amount of B or Ta to themulti-layered film. In general, B or Ta added to the multi-layered filmpromotes displacement of Cr atoms toward the grain boundaries. However,an excess amount of B or Ta greatly lowers the perpendicular magneticanisotropy; therefore, their amount is preferably several at %, at most.

The present inventors also studied the magnetic recording medium inwhich a Pt layer is used as the non-magnetic metal layer. However, itwas found that the Pt layer did not contribute to good characteristics.If a Pt alloy layer is used as the non-magnetic metal layer, theinterface between the Pt alloy layer and the CoCr-based alloy layer ismixed during sputtering at a low gas pressure. This leads to aninsufficient level of K_(s) (surface magnetic anisotropy energy). Inorder to use Pt non-magnetic metal layer, the production of themulti-layered magnetic film needs a high sputtering gas pressure inorder to achieve a large value of K_(s). On the other hand, it isnecessary to raise the degree of crystallization of CoCr-based alloy inorder to increase the value of K_(v). To this end, it is desirable toperform sputtering at a low gas pressure in order to prevent thesputtering gas from entering the alloy. These film-forming conditionsare incompatible with one another for the Pt non-magnetic metal layer.

The results of the present inventors' investigation revealed that alarge value of K_(s) can be obtained at a comparatively low gas pressureif the non-magnetic metal layer is composed mainly of Pd themulti-layered magnetic film 14 composed of ferromagnetic metal layersand non-magnetic metal layers each one layer of which is alternatelylaminated on top of one layer of the other kind. The magnetic recordingfilm 14, in which Pd is a major component in the non-magnetic metallayer, has high surface magnetic anisotropy, with the CoCr-based alloylayer keeping high crystal magnetic anisotropy.

Incidentally, the multi-layered magnetic film 14 is not specificallyrestricted in thickness. However, it should have an adequate thicknesscorresponding to the perpendicular magnetic anisotropy of themulti-layered magnetic film 14, because it is vulnerable to thermaldisturbance if it is excessively thin.

The perpendicular magnetic recording medium according to the presentinvention is described more in detail with reference to the followingexamples.

EXAMPLE 1

This example demonstrates the perpendicular magnetic recording mediumaccording to the present invention, in which the substrate is a glassdisk (76 mm in diameter) with a planarized surface suitable for HDD.This glass disk has a hole (an inside diameter of 15 mm) at its centerthrough which it is placed on to a spindle.

This glass disk was mounted on a sputtering apparatus and then heated atabout 100° C. to remove any adsorbed water. On the glass disk was formedan underlying layer (40 nm thick) from NiTa₃₇Zr₁₀ alloy. On theunderlying layer was sequentially formed a Pd film (5 nm thick), amulti-layered magnetic film, a Pd film (1 nm thick), and a carbonprotective film. The multi-layered magnetic film is composed of noblemetal layers and ferromagnetic metal layers each one layer of which arelaminated alternately on top of one layer of the other kind.

The multi-layered magnetic film is formed by using a rotary cathodedevice as shown in FIG. 2. The rotary cathode has three target cathodes22 which rotate along the same circle to form a film on the substrate 21placed right above the center of the circle.

If two target cathodes (A and B) are mounted on the rotary cathodedevice and they are subjected to discharging, it is possible to form amulti-layered film (composed of layers A and layers B which arelaminated alternately) in a short time (hereinafter “simultaneousdischarging method”).

This rotary cathode device gives rise to a multi-layered magnetic film31 shown in FIG. 3(a). This multi-layered magnetic film 31 is notcomposed of complete layers. That is, the noble metal layers 32 and theferromagnetic metal layers 33 are formed such that the position of theirinterface periodically moves. However, the multi-layered magnetic film31 may be regarded as lamellar because the circumference of the glassdisk is sufficiently large (50 mm or above) relative to the thickness ofeach layer (about 1 nm).

It is also possible to form the multi-layered structure as shown in FIG.3(b) if the targets A and B on the rotary cathode device are actuatedfor discharging alternately (hereinafter “alternate dischargingmethod”).

In general, the simultaneous discharging method is advantageous over thealternate discharging method in producing the multi-layered magneticfilm of periodic structure in a short time. Therefore, the simultaneousdischarging method was employed in this and following examples.

In this example, a recording medium of multi-layered structure having amulti-layered magnetic film composed of ferromagnetic metal layers ofCoCr₂₁ (referred to as CoCr₂₁ alloy layers) and noble metal layers of Pdwhich are laminated alternately on top of the other was prepared.(“Pd/CoCr₂₁ medium” hereinafter.)

In this example, a recording medium of multi-layered structure having amulti-layered magnetic film composed of ferromagnetic metal layers ofCoCr₂₁ (referred to as CoCr₂₁ alloy layers) and noble metal layers of Ptwhich are laminated alternately on top of the other was also preparedfor comparison. (“Pd/CoCr₂₁ medium” hereinafter.)

The multi-layered magnetic film was formed such that each CoCr₂₁ alloylayer is 2.0 nm thick and each noble metal layer is 0.2-1.4 nm thick,with the total thickness being 20 nm. The layer thickness d1 and d2, thethickness ratio d1/d2, and the laminating period were controlled byregulating the sputtering power onto the noble metal target and Co alloytarget, as well as the speed of the rotary cathode device. The substratewas preheated so that its temperature was 250° C. when the multi-layeredmagnetic film was formed.

In a comparative example, a perpendicular magnetic recording mediumwhich consists of a substrate, an underlying layer (40 nm thick) ofNiTa₃₇Zr₁₀ alloy, a Pd film (5 nm thick), a single-layered magnetic film(20 nm thick) of CoCr₂₁, a Pd film (1 nm thick), and a carbon protectivefilm was prepared (“CoCr₂₁ medium” hereinafter). The single-layeredmagnetic film is a CoCr₂₁ film having the axis of magnetization in theperpendicular direction. The substrate was heated at 250° C. immediatelybefore the magnetic film was formed as in the case of forming the Pd(Pt) /CoCr₂₁ medium.

The sputtering gas pressure affects the surface magnetic anisotropyenergy K_(s) of the multi-layered magnetic film and the crystal magneticanisotropy energy K_(v) of the CoCr₂₁ film (which is a single-layeredmagnetic film) as shown in FIG. 4. The noble metal layer in themulti-layered magnetic film is 0.8 nm thick.

The perpendicular magnetic anisotropy energy K_(u) in the multi-layeredmagnetic film medium depends on K_(s) and K_(v), as approximatelyrepresented by equation (1) below. (H. J. G. Draaisam et al., J. Magn.Magn. Mater. 66 (1987) 351-355)K _(u)·λ=2·K _(s) +t·K _(v)   (1)

In equation (1), K_(u) represents the perpendicular magnetic anisotropyenergy of the multi-layered magnetic film as a whole, K_(s) representsthe surface magnetic anisotropy energy (per unit area) in the interfacebetween the CoCr₂₁ alloy layer and the noble metal layer, and K_(v)represents the crystal magnetic anisotropy energy of the CoCr₂₁ alloylayer. T denotes the thickness of each CoCr₂₁ alloy layer, λ denotes theperiod of lamination, and 2·K_(s) denotes the surface magneticanisotropy energy (for two interfaces) due to one period in themulti-layered film.

The value of K_(v) (crystal magnetic anisotropy energy) includes theshape anisotropy of the film (−2πM_(s) ²) which occurs when therecording film is magnetized in one direction. As equation (1) suggests,the ratio of K_(v) to K_(u) increases in proportion to the thickness (t)of the CoCr₂₁ alloy layer.

In the present invention, the surface magnetic anisotropy energy wascalculated according to equation (1) under on the assumption that K_(v)of the CoCr₂₁ alloy layer in the multi-layered magnetic film is equal toK_(v) of the CoCr₂₁ film (20 nm thick) as a single-layered magneticfilm.

FIG. 4 suggests that the value of K_(s) increases as the sputtering gaspressure increases if Pt is used in the noble metal layer. By contrast,the value of K_(s) increases as the sputtering gas pressure decreases ifPd is used in the noble metal layer. On the other hand, the K_(v) valueof CoCr₂₁ alloy film rapidly decreases as the sputtering gas pressureincreases.

In order to achieve the object of the present invention, it is necessaryto form the multi-layered magnetic film which has high surfaceanisotropy energy K_(s) such that the CoCr₂₁ alloy layer keeps highcrystal magnetic anisotropy. FIG. 4 suggests that for the CoCr₂₁ alloylayer to have high perpendicular magnetic anisotropy, it is necessary toperform sputtering at a low sputtering gas pressure. If the noble metallayer and the CoCr₂₁ alloy layer are formed in the same sputteringchamber, it is necessary to obtain high surface magnetic anisotropyenergy under the condition of low gas pressure. This apparently suggeststhat it is desirable to use Pd for the noble metal layer constitutingthe multi-layered magnetic film.

The results shown in FIG. 4 are discussed in the following. In general,sputtering method for forming a film tends to cause sputtering gas (suchas Ar) to enter the film during the film-forming process. Therefore, thesputtering gas pressure should be as low as possible to give ahigh-purity alloy so long as discharging can be performed underacceptable conditions. The value of K_(v) (crystal magnetic anisotropyenergy) depends largely on the degree of crystallization of theCoCr-based alloy. It is a matter of course that the value of K_(v)(crystal magnetic anisotropy energy) increases as the sputtering gaspressure decreases.

As mentioned, it is desirable to increase the sputtering gas pressure inthe production of superlattice. In fact, U.S. Pat. No. 5,106,703(mentioned above) mentions that the coercive force increases if thesputtering gas pressure is increased, and Ar, as the sputtering gas, isreplaced by Xe or Kr. Usually, the coercive force is proportional withthe perpendicular magnetic anisotropy energy.

However, FIG. 4 indicates that the Pt/CoCr₂₁ medium and the Pd/CoCr₂₁medium behave greatly differently toward the sputtering gas pressure. Itis noted that the noble metal layer with Pd has a high value of K_(s)when the pressure of Ar sputtering gas is low.

Such a difference between Pd and Pt in the noble metal layer arises fromthe difference in their atomic weight (Pd=195.1, Pt=106.4). Themulti-layered structure requires that the layer interface (as the sourceof surface magnetic anisotropy) be formed neatly. To this end, it isnecessary to keep sufficiently low the kinetic energy of sputteringparticles sticking to the film such that sputtering particles impingeupon the film without destroying the layer interface.

Sputtering particles are scattered by collision with sputtering gas at ahigh pressure in the chamber, so that they lose their kinetic energy.However, Pt particles do not readily lose their kinetic energy unlessthe sputtering gas pressure is considerably high, because Pt has a muchlarger atomic weight than Ar. (Ar=40) By contrast, Pd particles readilylose their kinetic energy at a comparatively low sputtering gas pressurealthough the difference between Pd and Ar in atomic weight is not solarge.

The relation between sputtering gas pressure and sputtering rate wasinvestigated by using the same chamber as mentioned. It was found thatwhen the sputtering gas pressure was raised from 3 Pa to 10 Pa, thesputtering rate of Pt remained almost unchanged but the sputtering rateof Pd decreased by more than half. This result suggests that there aremore Pd particles which do not reach the substrate due to scattering bythe sputtering gas. This coincides with the above-mentioned prediction.

Sputtering at a low gas pressure is close to the condition under whichthe conventional single-layered CoCr-based alloy crystalline recordingfilm is formed. In this case, a good layer interface is readily obtainedbecause there is only a small amount of impurities in the multi-layeredmagnetic film. In the case where Pd is used to form the noble metallayer, the value of K_(s) is high when the gas pressure is low becausethe effect of making a good layer interface excels the effect ofdestroying the interface by the sputtering particles.

Incidentally, since Co has a smaller atomic weight (58.9) than noblemetal, Co sputtering particles are less likely to damage the interfacethan the noble metal sputtering particles. Sputtering with Xe or Kr(with an atomic weight of 131.2 and 83.8, respectively) would befavorable to Pt; however, Ar is advantageous costwise.

An experiment was carried out to see how the surface magnetic anisotropyenergy of the multi-layered magnetic film depends on the thickness ofthe noble metal layer in the case of CoCr₂₁ alloy medium in which thethickness of the CoCr₂₁ alloy layer is fixed at 2.0 nm. The result ofthe experiment is shown in FIG. 5. It is noted that the surfaceperpendicular magnetic anisotropy energy (K_(s)) increases with theincreasing thickness of the noble metal layer of Pd. It rapidlyincreases until the thickness reaches 0.6 nm, and it becomes saturatedwhen the thickness further increases from 0.8 nm to 1.0 nm. Thissuggests that an excessively thin noble metal layer (Pd) does notcontribute to the surface perpendicular magnetic anisotropy energyarising from the multi-layered film structure.

Why the surface perpendicular magnetic anisotropy energy is small whenthe thickness of the Pd layer is small may be explained as follows. (1)If the Pd layer is excessively thin, no good layer structure is formed.(2) There are no sufficient electrons to generate the magneticanisotropy energy if the Pd layer is excessively thin. (Magneticanisotropy is caused by electrons in the Pd layer.)

It was found that each of the CoCr₂₁ alloy layers have individuallydifferent coercive force if the Pd noble metal layer is made thickerthan 2.0 nm. This suggests that magnetic exchange interaction betweenCoCr₂₁ alloy layers becomes so small that domains in each layer canreverse independently. Under this situation, the multi-layered magneticfilms as a whole do not function as a single magnetic film. That is, itcannot be used as the recording magnetic film. The Pd noble metal layerin the multi-layered magnetic film should be thinner than 2.0 nm.

A probable reason why the surface magnetic anisotropy energy becomessaturated when the thickness of the Pd noble metal layer isapproximately 1.0 nm is that magnetization induced by the Pd noble metallayer is limited to about 0.5 nm (about two-atom layer) from theinterface between the CoCr₂₁ alloy layer and the Pd noble metal layercaused by Co.

The present inventors measured the magnitude of magnetization induced inthe Pd noble metal layer by the adjacent CoCr₂₁ alloy layer. It wasfound that average magnetization remains nearly constant at about 100kA/m when the Pd noble metal alloy is thinner than 1.0 nm but itdecreases as the thickness of the Pd noble metal layer exceeds 1.0 nm.It is considered that magnetization induced in the Pd noble metal layeris responsible for at least part of surface magnetic anisotropy energy.

Then, the present inventors prepared several samples of multi-layeredmagnetic film medium (Pt/CoCr₂₁ medium) which vary in the thicknesses ofthe Pd noble metal layer and the CoCr₂₁ alloy layer. The multi-layeredmagnetic film was tested for perpendicular magnetic anisotropy energyand saturation magnetization. The results are shown in Tables 1 and 2.

As a reference, the present inventors also prepared a CoCr₂₁ medium witha 20-nm thick single-layered magnetic film. It was found that thesingle-layered magnetic film has perpendicular anisotropy energy ofabout 0.8×10⁵ J/m³ and saturation magnetization of 250 kA/m. Thissingle-layered magnetic film gave an M-H curve with a squareness ratioof about 0.55. TABLE 1 Structure and perpendicular magnetic anisotropyenergy of multi-layered magnetic film in Pd/CoCr₂₁ medium (unit: ×10⁵J/m³) Thickness of Pd film (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.5 Thickness of0.3 0.70 1.54 3.42 3.65 3.15 2.68 2.35 CoCr₂₁ alloy 0.5 0.75 1.32 2.953.30 2.82 2.45 2.15 film (nm) 0.8 0.75 1.23 2.52 2.75 2.38 2.36 1.95 1.00.80 1.15 2.15 2.42 2.21 2.20 1.83 1.5 0.83 1.02 1.90 2.12 1.95 1.931.67 2.0 — 1.10 1.73 1.95 1.76 1.68 1.55 2.5 — 0.93 1.55 1.82 1.65 1.581.46 3.0 — — 1.51 1.66 1.52 1.46 1.42 4.0 — — 1.32 1.45 1.40 1.32 1.305.0 — — — — 1.32 1.33 1.20 6.0 — — — — 1.33 1.25 1.06

TABLE 2 Structure and saturation magnetization (M_(s)) of multi-layeredmagnetic film in Pd/CoCr₂₁ medium (unit: kA/m) Thickness of Pd film (nm)0.2 0.4 0.6 0.8 1.0 1.2 1.5 Thickness of 0.3 190 168 150 140 135 135 130CoCr₂₁ alloy 0.5 209 185 167 152 150 142 135 film (nm) 0.8 220 202 186178 168 163 160 1.0 222 210 192 182 175 163 158 1.5 230 218 205 193 190179 170 2.0 — 226 215 202 200 186 188 2.5 — 230 221 205 205 205 194 3.0— — 226 212 207 205 200 4.0 — — 230 225 226 209 210 5.0 — — — — 224 233215 6.0 — — — — 226 226 220

It is noted that the multi-layered magnetic film has much largerperpendicular anisotropy energy than the single-layered magnetic film ofCoCr₂₁ medium. It is noted from Table 2 that the multi-layered magneticfilm decreases in saturation magnetization due to the Pd layer.

For the perpendicular magnetic recording medium to have stablemagnetization, it is necessary that its M-H curve has a squareness ratioclose to 1. This can be checked in terms of a parameter of K_(u)/2πM_(s)² according to Shimazu et al. (Nippon Ouyou Jiki Gakkaishi, vol. 25, No.4-2, pp. 539-542). The reference pointed out that this parameter shouldessentially be greater than 3 for a practical magnetic recording medium(with magnetic particles with an approximate diameter of 10 nm) inconsideration of effect on it by thermal disturbance and demagnetizingfield. Since K_(u)/2πM_(s) ²=H_(k)/4πM_(s), this parameter representsthe magnitude ratio of the anisotropic magnetic field to thedemagnetizing field (H_(k) is a value of the physical property whichindicates how hard it is for magnetic particles to undergo magnetizationreversal). In other words, it is an index indicating how stable themagnetization is in the medium against the demagnetization field inducedwhen the recording bits are uniformly magnetized.

The CoCr₂₁ alloy medium referenced in this example has a value ofK_(u)/2πM_(s) ²=1.73. This value suggests that the squareness ratioapparently deviating from 1. This parameter was calculated also for themulti-layered magnetic film in this example. The results are shown inTable 3. TABLE 3 Structure and K_(u)/2πM_(s) ² value of multi-layeredmagnetic film in Pd/CoCr₂₁ medium (unit: none) Thickness of Pd film (nm)0.2 0.4 0.6 0.8 1.0 1.2 1.5 Thickness of 0.3 3.086 8.684 24.19 29.6427.51 23.40 22.13 CoCr₂₁ alloy film 0.5 2.733 6.138 16.83 22.73 19.9519.34 18.78 (nm) 0.8 2.466 4.798 11.59 13.81 13.42 11.14 12.12 1.0 2.5834.150 9.282 11.63 11.49 13.18 11.67 1.5 2.497 3.416 7.196 9.058 8.5979.587 9.197 2.0 — 3.428 5.956 7.606 7.003 7.729 6.980 2.5 — 2.798 5.0516.893 6.249 5.984 6.174 3.0 — — 4.705 5.878 5.646 5.529 5.650 4.0 — —3.971 4.559 4.362 4.810 4.692 5.0 — — — — 4.187 4.257 4.132 6.0 — — — —4.144 3.895 3.486

It is noted from Table 3 that the multi-layered magnetic film has aconsiderably large value of K_(u)/2πM_(s) ² because the surface magneticanisotropy enhances the perpendicular magnetic anisotropy energy and thePd layer suppresses the average magnetization of the multi-layered filmas a whole.

For the multi-layered magnetic film to be a suitable perpendicularmagnetic recording medium, it should have perpendicular magneticanisotropy energy, saturation magnetization, and K_(u)/2πm_(s) ² valueas desired, as well as good recording characteristics.

Thus, the present inventors tested the perpendicular magnetic recordingmedium in this example so as to realize low recording noise for thestructure of the multi-layered magnetic film.

The test procedure consists of recording a continuous pattern with arecording density of 300 kFCI on each sample of the perpendicularmagnetic recording medium and reproducing the continuous pattern tomeasure the integral value of recording noise.

The perpendicular magnetic recording medium for recording andreproducing tests has an FeTaC soft magnetic layer (400 nm thick) onthat side of the NiTaZr underlying layer which faces the substrate.Recording was accomplished by using a single pole type head, andreproducing was accomplished by using a GMR sensor mounted on the samehead.

It was found that recording noise depends on the ratio of d1/d2 (whered1 denotes the thickness of the CoCr₂₁ alloy layer and d2 denotes thethickness of the Pd noble metal layer) as shown in FIG. 6. In FIG. 6,recording noise is given in terms of relative noise strength comparedwith the noise of the CoCr₂₁ medium with the single-layered magneticfilm (for reference). The noise strength of each medium is normalizewith respect to the low-frequency signal strength.

It is noted that the relative noise strength decreases as the relativethickness of the CoCr₂₁ alloy layer increases (or it is about 1 whend1/d2 is about 1.5). A probable reason why the recording noise is largewhen the d1/d2 is small is that there is large magnetic exchangeinteraction between particles and there is large domain walldisplacement at the time of recording. This result suggests that themulti-layered magnetic recording medium has a low level of recordingnoise as in the CoCr₂₁ medium with the single-layered magnetic film ifit is constructed such that the CoCr₂₁ alloy layer is more than 1.5times thick than the Pd noble metal layer. It was necessary to keep thesubstrate at about 250° C. when layers are formed thereon.

The values of K_(u)/2πM_(s) ² (smaller than 10) in Table 3 are plottedagainst the thickness of the Pd noble metal layer, with the thickness ofthe CoCr₂₁ alloy layer varied, as shown in FIG. 7. All layer thicknessconditions plotted in FIG. 7 satisfy a condition that its d1/d2 value islarger than 1.5 such that the magnetic exchange interaction betweenparticles is sufficiently reduced. It is noted from FIG. 7 that all ofthe multi-layered magnetic recording medium have larger K_(u)/2πM_(s) ²values than the CoCr₂₁ medium for reference. This suggests that themulti-layered magnetic film medium is more thermally stable than thesingle-layered magnetic film medium.

In FIG. 7, all the plotted points are within the thick broken line. Ifthe Pd noble metal layer is excessively thin, the surface magneticanisotropy is insufficient. If the Pd noble metal layer is excessivelythick, the density in the interface decreases because it is necessarythat the CoCr₂₁ alloy layer should also be thick. Consequently, thethickness of the Pd noble metal layer which gives the maximum value ofK_(u)/2πM_(s) ² is 0.8 to 1.0 nm, which is equivalent to the thicknessat which the surface magnetic anisotropy energy becomes saturated.

The present inventors examined the relation between the K_(u)/2πM_(s) ²value and the thermal demagnetizing factor S of the multi-layeredmagnetic film medium in this example by recording a continuous pattern(20 kFCI) on each sample of the medium and then measuring the amount ofloss in the reproduced signal. The thermal demagnetizing factor S is thecoefficient of magnetic viscosity which relates the rate of signal lossto log₁₀ (time). It was determined by plotting the amount of signal loss(measured for a period of 1000 seconds) on a graph in which the abscissarepresents log₁₀ (time) and the ordinate represents the rate of signalloss. An example of measurements is shown in FIG. 8.

In FIG. 9, the thermal demagnetizing factor S is plotted against theK_(u)/2πM_(s) ² value of each medium. FIG. 9 indicates that the thermaldemagnetizing factor S approaches zero when K_(u)/2πM_(n) ² increases.In other words, thermal demagnetization is practically negligible forthe K_(u)/2πM_(s) ² value exceeding 6. The result of this investigationsuggests that the K_(u)/2πM_(s) ² value should be greater than 6 so thatthe perpendicular magnetic recording medium permits recorded signals toremain thermally stable.

It is noted from FIG. 7 that this condition is satisfied when thethickness (d2) of the Pd noble metal layer is larger than 0.6 nm. If thePd alloy layer is excessively thick (or thicker than 2.0 nm), magneticexchange interaction between CoCr-based alloy layers is insufficient andhence CoCr-based alloy layers would have different coercive forceindependently. Therefore, the thickness (d2) of the Pd noble metal layershould be from 0.6 nm to 2.0 nm, and preferably, from 0.8 nm to 1.2 nm.

The data in FIG. 7 is rearranged in FIG. 10. In FIG. 10, theK_(u)/2πM_(s) ² value is plotted against the ratio of d1/d2 (where d1 isthe thickness of the CoCr-based alloy layer and D2 is the thickness ofthe Pd noble metal layer). It is noted that the K_(u)/2πM_(s) ² value isnot large when the thickness of the Pd layer is 0.2 nm or 0.4 nm. Theabove-mentioned condition is not met when the ratio of d1/d2 is smallerthan 1.5 (requirement for good signal quality). On the other hand, forthe Pd layer with a thickness of 0.6 nm, the K_(u)/2πM_(s) ² valuesuddenly increases and the condition that d1/d2 is larger than 1.5 andK_(u)/2πM_(s) ² is larger than 6 is met. However, with an excessivelylarge d1/d2, the condition that K_(u)/2πM_(s) ² is larger than 6 is notmet because the laminate period decreases (and hence the number ofinterfaces between the Co layers and Pd layers decreases), and thesurface magnetic anisotropy energy decreases. FIG. 10 shows limited datafor the Pd layer with a thickness up to 0.8 nm. Beyond this thickness,the value of K_(u)/2πM_(s) ² tends to decrease. It is concluded fromFIG. 10 that the allowable maximum value of d1/d2 is 4.

EXAMPLE 2

This example demonstrates a magnetic recording medium in which theferromagnetic metal layer is formed from a material having a large valueof Kv in place of the CoCr-based alloy used in Example 1. In Example 1,the CoCr layer as the ferromagnetic metal layer is inherently limited incrystal perpendicular magnetic anisotropy energy K_(v) and hence therange of d1/d2 for keeping the K_(u)/2πM_(s) ² value exceeding 6 (withthe maximum being about 10) is rather narrow. The advantage of therecording medium in this example is that the problem with thermaldemagnetization can be avoided even in the case where the diameter ofmagnetic particles is reduced further in order to decrease recordingnoise.

The multi-layered magnetic recording medium prepared in this exampleconsists of a substrate, an underlying layer of NiTa₃₇Zr₁₀ alloy (40 nmthick), a Pd film (5 nm thick) a multi-layered magnetic film ofPd/CoCr₂₁Pt₁₄, a Pd film (1 nm thick), and a carbon protective film,which are sequentially placed on top of the other. The multi-layeredmagnetic film of Pd/CoCr₂₁Pt₁₄ is referred to as “Pd/CoCr₂₁Pt₁₄ film”hereinafter. The multi-layered magnetic recording medium is referred toas “Pd/CoCr₂₁Pt₁₄ medium” hereinafter. Incidentally, the total thicknessof the multi-layered magnetic film is about 20 nm, and the substrate washeated to 250° C. immediately before film lamination.

A comparative sample of perpendicular magnetic recording medium was alsoprepared, which consists of a substrate, an underlying layer ofNiTa₃₇Zr₁₀ alloy (40 nm thick), a Pd film (5 nm thick), a single-layeredmagnetic film of CoCr₂₁Pt₁₄, a Pd film (1 nm thick), and a carbonprotective film, which are sequentially placed on top of the other. Thesingle-layered magnetic film of CoCr₂₁Pt₁₄ is referred to as “CoCr₂₁Pt₁₄film” hereinafter. The single-layered magnetic recording medium isreferred to as “CoCr₂₁Pt₁₄ medium” hereinafter. As in the Pd/CoCr₂₁Pt₁₄medium, the substrate was heated to 250° C. immediately before filmdeposition.

The CoCr₂₁Pt₁₄ film was found to have perpendicular anisotropy energy ofabout 1.6×10⁵ J/m³ and saturation magnetization of 250 kA/m. TheCoCr₂₁Pt₁₄ film is quite similar in saturation magnetization to theCoCr₂₁ alloy layer (in Example 1) which does not contain Pt. However,the CoCr₂₁Pt₁₄ single-layered magnetic film increases in crystalmagnetic anisotropy.

As shown above, the incorporation of Pt into the ferromagnetic metallayer of CoCr-based alloy is an effective way for increasing themagnetic anisotropy of the magnetic film. The comparison of thePd/CoCr₂₁Pt₁₄ medium with the Pd/CoCr₂₁ medium revealed that the formerhas larger perpendicular magnetic anisotropy energy than the latter butthere is no significant difference between them in the averagesaturation magnetization of the entire multi-layered film. The incrementof perpendicular magnetic anisotropy is due mostly to crystal magneticanisotropy energy but very little to surface magnetic anisotropy energy.The CoCr₂₁Pt₁₄ alloy is suitable for use as the ferromagnetic metallayer of the multi-layered magnetic film medium, because it increases incrystal magnetic anisotropy owing to the incorporation of Pt even if itis made into multi-layered thin film.

The multi-layered magnetic film medium (Pd/CoCr₂₁Pt₁₄ medium) in thisexample was examined for a relation between d1/d2 and recording noise.It was found that recording noise can be suppressed if d1/d2 is largerthan 1.5 as in Example 1.

Samples of Pd/CoCr₂₁Pt₁₄ medium varying in their layer thickness weremeasured for K_(u)/2πM_(s) ² values. The results are shown in FIG. 11.In comparison of FIG. 11 with FIG. 7 (in term of the Pd/CoCr₂₁ medium),it is noted that the Pd/CoCr₂₁Pt₁₄ medium has larger K_(u)/2πM_(s) ²values than the Pd/CoCr₂₁ medium, with the maximum value being about 12when d2 is in the range of 0.8 to 1.0 nm. It is also noted that the Pdnoble metal layer should have a thickness (d2) larger than 0.6 nm so asto meet the condition that K_(u)/2πM_(s) ² of the CoCr₁₅Pt₁₄B₂ alloy isgreater than 6 (as the index for thermal stability). If the Pd alloylayer is excessively thick (or thicker than 2.0 nm), magnetic exchangeinteraction between CoCr-based alloy layers is insufficient, and henceCoCr-based alloy layers would have different coercive forceindependently.

Samples of Pd/CoCr₂₁Pt_(x) medium were examined for perpendicularmagnetic anisotropy, with the amount of Pt varying in the ferromagneticmetal layer of CoCr-based alloy. The CoCr₂₁Pt_(x) alloy layer is 1.6 nmthick, and the Pd noble metal layer is 0.8 nm thick.

It was found that the perpendicular magnetic anisotropy energy linearlyincreases in proportion to the amount of Pt added (up to 10 at %).However, the perpendicular magnetic anisotropy remained almost unchangedbeyond 10 at %, and recording noise begins to increase beyond 16 at %.This indicates that incorporation with Pt increases magnetic exchangeinteraction between particles. It is concluded from the foregoing thatthe ferromagnetic metal layer of CoCr-based alloy should be incorporatedwith Pt in an amount from 10 at % to 16 at %.

EXAMPLE 3

This example demonstrates a multi-layered magnetic film recording medium(“Pd/CoCr₁₅Pt₁₄B₂ medium” hereinafter) which was prepared in thefollowing manner. First, a substrate was coated sequentially with anunderlying layer (40 nm thick) of NiTa₃₇Zr₁₀ alloy and a Pd film (5 nmthick). With the substrate heated to 250° C., the Pd film was coatedwith an initial layer (3 nm thick) of CoCr₄₀ alloy and subsequently witha multi-layered magnetic film of Pd/CoCr₁₅Pt₁₄B₂ (“Pd/CoCr₁₅Pt₁₄B₂ film”hereinafter) by using the rotary cathode device. After the multi-layeredmagnetic film had been formed, a heat treatment was performed at 350° C.or above for 10 seconds. After cooling, a Pd film (1 nm thick) and acarbon protective film were formed sequentially.

In general, the crystal magnetic anisotropy energy (K_(v)) increases butthe effect of reducing the magnetic exchange interaction betweenparticles at Cr grain boundaries is lost as the Cr concentrationdecreases in the CoCr-based alloy. According to the conventionaltechnology, the minimum Cr concentration was 20% which decreases themagnetic exchange interaction between particles without increase inrecording noise.

The present inventors found that the magnetic exchange interactionbetween particles decreases to a level suitable for magnetic recordingif heat treatment is performed at 350° C. or above for 10 seconds or soafter the multi-layered magnetic film has been formed.

The reason why the magnetic exchange interaction between particlesdecreases upon a heat treatment may be understood as follows. The CoCr₄₀initial layer just under the multi-layered magnetic film contains Cratoms which, during reheating, diffuse through the grain boundaries ofmagnetic particles in the direction perpendicular to the film surface.

A comparative sample of single-layered magnetic film medium(“CoCr₁₅Pt₁₄B₂ medium” hereinafter) was prepared in the followingmanner. A substrate was coated sequentially with an underlying layer (40nm thick) of NiTa₃₇Zr₁₀ alloy and a Pd film (5 nm thick). With thesubstrate heated to 250° C., the Pd film was coated with an initiallayer (3 nm thick) of CoCr₄₀ alloy and subsequently with asingle-layered magnetic film of CoCr₁₅Pt₁₄B₂ (“CoCr₁₅Pt₁₄B₂ film”hereinafter). After the CoCr₁₅Pt₁₄B₂ film had been formed, a heattreatment was performed at 350° C. or above for 10 seconds. Aftersufficient cooling, a Pd film (1 nm thick) and a carbon protective filmwere formed sequentially.

The CoCr₁₅Pt₁₄B₂ alloy film was found to have large perpendicularanisotropy energy of about 2.5×10⁵ J/m³ and saturation magnetization of320 kA/m (which is larger than that of the CoCr₁₅ alloy film) due to thesmall Cr content (15%). Therefore, the recording medium has a largedemagnetizing field, and the single-layered film of CoCr₁₅Pt₁₄B₂ has aK_(u)/2πM_(s) ² value of 3.9. In other words, the single-layered film ofCoCr₁₅Pt₁₄B₂ differs very little from the CoCr₂₁Pt₁₄ film with a largeCr content (without heat treatment, referred to Example 2) in terms ofK_(u)/2πM_(s) ² (thermal stability), because the increased perpendicularmagnetic anisotropy is cancelled by the increased demagnetizing field.

The Pd/CoCr₁₅Pt₁₄B₂ medium, which underwent heat treatment after themulti-layered magnetic film had been formed, was examined forperpendicular magnetic anisotropy energy (K_(u)) and saturationmagnetization (M_(s)), with the thickness of the multi-layered magneticfilm being adequately varied. The results are shown in Tables 4 and 5.TABLE 4 Structure and perpendicular magnetic anisotropy energy (K_(u))of multi-layered magnetic film in Pd/CoCr₂₁Pt₁₄B₄ medium with reheating(unit: ×10⁵ J/m³) Thickness of Pd film (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.5Thickness of 0.3 2.02 2.92 5.56 6.87 6.45 5.72 4.75 CoCr₂₁Pt₁₄B₂ 0.51.98 2.65 5.43 6.15 5.92 5.30 4.67 alloy film 0.8 2.03 2.55 4.85 5.645.26 4.95 4.30 (nm) 1.0 2.15 2.42 4.35 5.45 4.98 4.75 4.05 1.5 2.10 2.303.98 4.81 4.45 4.30 3.68 2.0 — 2.32 3.78 4.33 4.07 3.77 3.30 2.5 — 2.323.66 4.05 3.80 3.85 3.35 3.0 — — 3.42 3.75 3.58 3.42 3.32 4.0 — — 3.153.45 3.20 3.35 3.04 5.0 — — — — 3.25 3.05 3.12 6.0 — — — — 3.04 3.052.95

TABLE 5 Structure and saturation magnetization (M_(s)) of multi-layeredmagnetic film in Pd/CoCr₂₁Pt₁₄B₄ medium with reheating (unit: kA/m)Thickness of Pd film (nm) 0.2 0.4 0.6 0.8 1.0 1.2 1.5 Thickness of 0.3240 222 175 164 150 152 143 CoCr₂₁Pt₁₄B₂ 0.5 275 248 205 190 175 170 165alloy film 0.8 284 268 231 215 210 195 167 (nm) 1.0 290 272 244 226 210208 190 1.5 298 280 270 250 237 226 220 2.0 — 290 272 270 250 242 2302.5 — 300 285 268 265 256 243 3.0 — — 285 275 270 263 260 4.0 — — 296295 280 270 262 5.0 — — — — 293 272 275 6.0 — — — — 295 292 284

It is noted from that the multi-layered magnetic film has higher valuesof both K_(u) and K_(s) as compared with that in Example 2 for the samelayer thickness. On the other hand, the multi-layered magnetic film haslarger surface magnetic anisotropy energy (K_(s)) as compared with thatof Pd/CoCr₂₁ medium in Example 1 and that of Pd/CoCr₂₁Pt₁₄ medium inExample 2, both by more than 50%, for the same structure. This may beattributable to the increased Co concentration in the Co alloy layer andthe increased area of Pd/Co interface.

As in Example 1 (FIG. 7) and Example 2 (FIG. 11), the K_(u)/2πM_(s) ²values are plotted against d2, with d1 varied such that d1/d2 is greaterthan 1.5 (for sufficiently small magnetic exchange interaction betweenparticles). The result is shown in FIG. 12. This result is quite similarto that in Example 2. However, the recording medium in this example hasmore preferable properties than that in Example 2. One of suchproperties is good thermal stability for high recording density.

What K_(u)/2πM_(s) ² values represent is thermal stability whichmanifests itself when magnetization in the medium is oriented in thesame direction or when the recording density is low. With a lowrecording density, the Pd/CoCr₁₅Pt₁₄B₂ medium in this example is almostidentical in thermal stability to the medium in Example 2, due to theeffect of the demagnetizing field. However, as the recording densityincreases, the medium in this example, which has a larger value ofK_(u), has the advantage in thermal stability. Another of the preferableproperties is the high strength of reproduced signals. In other words,the Pd/CoCr₁₅Pt₁₄B₂ medium in this example has larger magnetization thanthe medium in Example 2, which provides a larger output of reproducedsignals.

The present inventors closely studied heat treatment performed after theCoCr₁₅Pt₁₄B₂ film has been formed. It was found that reheating does notcut off the magnetic exchange interaction between particles in themulti-layered magnetic film if the Cr content is lower than 12 at %.This result suggests that it is necessary that the Cr content in the Coalloy should be higher than 12 at %.

It was found that Cu in place of Cr is also useful as the additive tothe initial layer. The amount of Cu in the initial layer should belarger than that in the multi-layered magnetic film. An amount more than30 at % is necessary for reheating to produce a remarkable effect.

In addition, this example is characterized by that the Co alloy isincorporated with a small amount (2 at %) of B. The incorporation of Breduces the size of magnetic particles by about 10%. It is known that,in general, such elements as B and Ta added in small quantities to theCoCr-based alloy promote segregation of Cr atoms on the grain boundaryand reduce the size of crystal grains. It was confirmed that this iseffective also in the multi-layered magnetic film medium. More detailsare discussed in Example 4.

EXAMPLE 4

This example demonstrates a multi-layered magnetic film recording mediumof the same structure as in Example 1, except that the multi-layeredfilm is incorporated with boron (B). This recording medium has aferromagnetic metal layer of CoCr₂₁Pt₁₄B_(x) (1.8 nm thick) and a noblemetal layer of PdB_(x) (0.8 nm thick). The multi-layered magnetic filmhas a total thickness of 20 nm.

The amount of B added is expressed in terms of x at %. The layerscontaining B were prepared in the same way as in Example 1. Thesubstrate was heated to 250° C. after the underlying layer had beenformed. With x varied from 0 at % to 5 at %, the multi-layered magneticfilm was examined for magnetic properties and crystal grain sizes. Theresults are shown in Table 6. Incidentally, the measurements of crystalgrain size were carried out by using a scanning tunnel electronmicroscope (TEM) and crystal structure analysis. TABLE 6 Magneticproperties and crystal grain size of multi-layered magnetic thin film(20 nm thick) composed of PdB_(x) (0.8 nm) and CoCr₂₁Pt₁₄B_(x) (1.8 nm)Perpendicular Saturation magnetic Crystal magnetization Coerciveanisotropy grain Amount of B (M_(s)) force (H_(c)) energy (K_(u)) size(D) added (x at %) (kA/m) (kA/m) (J/m³) (nm) 0 202 415 2.42 × 10⁵ 11.5 1200 376 2.33 × 10⁵ 10.5 2 195 352 2.04 × 10⁵ 10.2 3.5 191 310 1.95 × 10⁵9.7 5 185 292 1.81 × 10⁵ 9.2

It is noted from Table 6 that the incorporation of B into thenon-magnetic metal layer (Pd) slightly decreases the crystal grain sizeas well as K_(u) and H_(c). Because of the decrease in K_(u) and thegrain size, the perpendicular magnetic recording medium is subject tothermal disturbance and hence is poor in thermal stability.Nevertheless, it has a low level of recording noise owing to thereduction in grain size.

In this example, the incorporation of 5 at % B did not cause appreciablethermal demagnetization. Rather, the incorporation of B reduces thegrain size and adjust K_(u) and M_(s) to adequate values suitable forperpendicular magnetic recording. This in turn improves resistance tothermal disturbance and contributes to the multi-layered magnetic filmsuitable for the perpendicular magnetic recording medium.

The same effect as mentioned above was also obtained when B was replacedby tantalum (Ta).

As mentioned above, the perpendicular magnetic recording medium consistsof a substrate and a multi-layered magnetic film, with or without anunderlying layer interposed between them. The multi-layered magneticfilm is composed of ferromagnetic metal layers containing Co andnon-magnetic metal layers containing Pd, each one layer of which arelaminated alternately on top of one layer of the other. Theferromagnetic metal layers and the non-magnetic metal layers have athickness of d1 and d2, respectively, with the ratio of d1/d2 being inthe range of 1.5 to 4.0. This specific layer structure reduces themagnetic exchange interaction between magnetic particles in the magneticrecording film and increases the perpendicular magnetic anisotropyenergy (K_(u)) relative to the demagnetizing field of the recordingmedium. As a result, the perpendicular magnetic recording medium isstable against thermal disturbance and has a low level of recordingnoise.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

1. A process for producing a perpendicular magnetic recording medium,said process comprising a step of forming an underlying layer on asubstrate, a step of forming on said underlying layer an initial layerof Co alloy containing Cr or Cu, a step of forming on said initial layera multi-layered magnetic film composed of non-magnetic metal layerscontaining Pd and ferromagnetic metal layers containing Co which arelaminated alternately on top of the other, a step of heat treatment at atemperature higher than 350° C. which is performed after saidmulti-layered magnetic film has been formed, and a step of forming acarbon protective film on said multi-layered magnetic film.
 2. A processfor producing a perpendicular magnetic recording medium according toclaim 1, wherein said ferromagnetic metal layers contain less than 20 at% of Cr.
 3. A process for producing a perpendicular magnetic recordingmedium according to claim 1, wherein said ferromagnetic metal layerscontain mores than 12 at % of Cr.
 4. A process for producing aperpendicular magnetic recording medium according to claim 1, whereinsaid initial layer of Co alloy contain more than 30 at % of Cr or Cu.